JP2011182613A - Power conversion system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that, when energy stored in an input side inductor W1 is supplied to a power source other than a high voltage battery 12, power needs to be supplied via the high voltage battery 12. <P>SOLUTION: An input terminal of an inverter IV is connected to the high voltage battery 12 via each of a main switch Swm, a sub-switch Sws and the input side inductor W1. An inductor W2 for supplying the energy is magnetically coupled to the input side inductor W1. A capacitor 24 is connected to the inductor W2 for supplying the energy via a diode 22 that takes a voltage impressed when the sub-switch Sws is turned off as a forward direction. This structure enables the energy stored in the input side inductor W1 to be recovered by the capacitor 24 by turning off the sub-switch Sws after the main switch Swm is turned into an ON state. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention provides a series connection of a high potential side switching element for connecting the first inductor to the positive electrode side of a DC power supply and a low potential side switching element for connecting the first inductor to the negative electrode side of the DC power supply. The present invention relates to a power conversion system including a body.

  As this type of power conversion system, for example, as seen in Patent Document 1 below, an electric path including an inductor and an electric path not including an inductor are provided between an upper arm of an inverter and a DC power source. A device having a switching element in each path has also been proposed. According to this, with the switching elements of the pair of electric paths turned off, the switching element to be turned on is changed from the switching element (low potential side switching element) of the lower arm to the switching element (high potential side switching) of the upper arm. Switching to the element) enables zero voltage switching. Further, by turning on the switching element of the electric path including the inductor prior to the turning-on operation of the switching element of the electric path not including the inductor, the ON operation of the switching element of the electric path not including the inductor is switched to zero current. It is described that it can be.

  Further, it is also described that the system includes a loop path for recovering energy stored in the inductor to a DC power source by turning off the switching element of the electrical path including the inductor.

Japanese National Patent Publication No. 11-506599

  However, since the above system recovers the energy stored in the inductor to the DC power source, for example, when it is desired to supply electrical energy to a different power source, the energy from the DC power source to the other power source is determined. It is necessary to provide a separate route for transmitting.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a power conversion system capable of recovering energy stored in the inductor by a member separate from the energy supply source to the inductor. There is to do.

  Hereinafter, means for solving the above-described problems and the operation and effect thereof will be described.

  The invention according to claim 1 is a high potential side switching element for connecting the first inductor to the positive electrode side of the DC power supply, and a low potential side switching element for connecting the first inductor to the negative electrode side of the DC power supply. In a power conversion system including a series connection body, a free wheel diode is connected in antiparallel to one of the high potential side switching element and the low potential side switching element, and among the pair of electrodes of the DC power supply A first electric path connecting the electrode side connected to the first inductor by the other switching element and the other switching element and having a second inductor and first opening / closing means; and a pair of electrodes of the DC power supply And the other switching element connected to the first inductor by the other switching element. Prior to switching from the ON state of the one switching element to the ON state of the other switching element, the first opening / closing means is closed while connecting the element and having the second opening / closing means And an opening / closing operation means for opening the second opening / closing means, closing the second opening / closing means and opening the first opening / closing means after the switching, and a second magnetically coupled to the second inductor. Three inductors, and rectifying means that uses a voltage applied to the third inductor when the first opening / closing means is switched to the open state by the opening / closing operation means as a forward voltage, the third inductor includes: It is connected to a power storage means different from the DC power source through a rectifying means.

  When the first opening / closing means is switched to the open state by the opening / closing operation means, the loop path for allowing the energy stored in the second inductor to flow is opened. However, at this time, the energy stored in the second inductor flows out to the power storage means via the third inductor and the rectifying means. For this reason, the energy stored in the second inductor can be recovered by the power storage means different from the DC power source.

  According to a second aspect of the present invention, in the first aspect of the present invention, the power storage means is a power source for a cooling device that cools at least one of the power conversion system and the DC power source.

  The high-potential side switching element, the low-potential side switching element, the first inductor, the second inductor, the free wheel diode, the DC power source, and the like generate heat when the power conversion system is in a driving state and a current flows through them. For this reason, the required degree of the cooling capacity by the cooling device increases as the power conversion system is driven (the high potential side switching element and the low potential side switching element are driven) and the current flowing through the system increases. On the other hand, the power supplied to the power storage means tends to increase as the power conversion system is driven and the current flowing therethrough increases. For this reason, in the said invention, when there is much required electric power of a cooling device, big electric power can be supplied to an electrical storage means.

  The invention according to claim 3 is the invention according to claim 1 or 2, wherein the high potential side switching element and the low potential side switching element constitute an in-vehicle high voltage system, and the power storage means It constitutes an in-vehicle low voltage system insulated from the in-vehicle high voltage system.

  In the above invention, since the power of the DC power source can be output to the power storage means via the second inductor and the third inductor, the power of the DC power source constituting the high voltage system is not passed through a dedicated converter or the like. It can be supplied to power storage means constituting the low voltage system.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, when the other switching element is switched to the on state, the shoot-through control means that turns both on, After the both are turned on by the shoot-through control means, the one switching element after the point when the total current flowing through the one switching element and the freewheel diode connected in reverse parallel to the switching element becomes zero And an off operation means for performing an off operation.

  In the above invention, since the shoot through control means and the off operation means are provided, it is possible to suitably suppress or avoid the current flowing through the free wheel diode and the recovery current flowing through the free wheel diode. For this reason, it is possible to suppress a change in current when the other switching element is turned on or one of the switching elements is turned off.

  According to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, the one switching element is configured such that an electric current flows between the pair of terminals by switching the input terminal and the output terminal. It is allowed to flow in the direction.

  In the said invention, it can avoid suitably that an electric current flows into a freewheel diode by shoot through control.

1 is a system configuration diagram according to a first embodiment. FIG. The time chart which shows the operation method of the power converter circuit concerning the embodiment. The system block diagram concerning 2nd Embodiment.

(First embodiment)
Hereinafter, a first embodiment in which a power conversion system according to the present invention is applied to a hybrid vehicle will be described with reference to the drawings.

  The illustrated electric motor 10 is a three-phase rotating machine as an in-vehicle main machine, and specifically may be a synchronous machine, for example. The electric motor 10 is connected to a smoothing capacitor 13 and a high voltage battery 12 via an inverter IV. Here, the inverter IV is configured by connecting in parallel three serially connected bodies of a power switching element Swp on the high potential side and a power switching element Swn on the low potential side. And the connection point of each of these series connection bodies is connected to each phase of the electric motor 10, respectively. Between the input / output terminals (between the drain and source) of the high potential side power switching element Swp and the low potential side power switching element Swn, there is a high potential side freewheel diode Fwp and a low potential side freewheel diode. The cathode and anode of Fwn are connected.

  A main switch Swm, a sub-switch Sws, and a primary coil of the transformer T (input-side inductor W1) are connected between the high-potential-side power switching element Swp of the inverter IV and the positive electrode of the high-voltage battery 12. Has been.

  The power switching elements Swp, Swn, the main switch Swm, and the sub switch Sws of the inverter IV are cooled by the cooling liquid CL that flows in the cooling passages in contact with them. The cooling liquid CL is circulated by the cooling device 26. The cooling device 26 includes an electric motor or the like, and uses the electric energy stored in the capacitor 24 as driving energy. The energy stored in the input-side inductor W1 is supplied to the capacitor 24 through the secondary coil (energy supply inductor W2) of the transformer and the diode 22 that are magnetically coupled to the capacitor.

  The control device 30 is a control device that uses a low-voltage battery 32 as a power source. The control device 30 controls operation signals gup, gvp, gwp for operating the power switching element Swp for each of the U-phase, V-phase, and W-phase of the inverter IV based on detection values of various sensors (not shown), Operation signals gun, gvn, and gwn for operating the switching element Swn are generated and output. Thereby, the switching elements Swp and Swn are operated by the control device 30. In addition, the control device 30 outputs operation signals gm and gs to the main switch Swm and the sub switch Sws, respectively. Accordingly, the main switch Swm and the sub switch Sws are operated by the control device 30.

  The inverter IV, the high voltage battery 12 and the like constitute an in-vehicle high voltage system, while the control device 30, the low voltage battery 32, the cooling device 26 and the like constitute an in-vehicle low voltage system. The high voltage system and the low voltage system are insulated from each other. For this reason, the operation signals gup, gvp, gwp, gun, gvn, gwn, gm, gs are output to a high voltage system via an insulating means such as a photocoupler.

  The power switching elements Swp and Swn, the main switch Swm, and the sub switch Sws are all N-channel super junction MOS field effect transistors. A body diode is formed in these. That is, for example, a free wheel diode Fwp is formed in the power switching element Swp on the high potential side, and a free wheel diode Fwn is formed in the power switching element Swn on the low potential side.

  The super junction MOS field effect transistor has a very high rate of decrease after the recovery current of the body diode has changed from increasing to decreasing. For this reason, a surge due to a change in the recovery current may be particularly large. Therefore, in this embodiment, this problem is dealt with by the switching operation shown in FIG. In addition, at this time, the capacitor 24 is charged by the energy stored in the input-side inductor W1 along with the switching operation.

  FIG. 2 shows a switching operation method according to the present embodiment. Specifically, FIG. 2A shows the transition of the operation mode of the sub switch Sws, FIG. 2B shows the transition of the operation mode of the main switch Swm, and FIG. FIG. 2D shows the transition of the operation mode of the low-potential side power switching element Swn in phase with the power switching element Swp of FIG. 2C. FIG. 2 (e) shows the transition of the current iW1 flowing through the input-side inductor W1 (see FIG. 1 for the sign), and FIG. 2 (f) shows the transition of the current iW2 flowing through the energy supply inductor W2 (the sign is shown). 1 (see FIG. 1), FIG. 2 (g) shows the transition of the current is flowing through the sub switch Sws etc. (including the body diode), and FIG. 2 (h) shows the main switch Swm etc. (including the body diode). ) Shows the transition of the current im flowing through. Further, FIG. 2 (i) shows the transition of the current ip flowing through the power switching element Swp on the high potential side and the like (including the body diode: see FIG. 1 for the reference), and FIG. 2 (j) shows the low potential side. FIG. 2 (k) shows the transition of the current in flowing through the power switching element Swn and the like (including the body diode: see FIG. 1). FIG. 2 (k) shows the voltage V1 at the connection point between the sub switch Sws and the input-side inductor W1. FIG. 2 (l) shows the transition of the voltage V2 between the power switching elements Swp and Swn. In the following, it is assumed that a current flows from the inverter IV to the electric motor 10 in the target phase.

  As illustrated, the period T0 is a period in which the high-potential side power switching element Swp is in an off state and the low-potential side power switching element Swn is in an on state. Here, in the period T1, when the high-potential side power switching element Swp is switched to the ON state, the low-potential side power switching element Swn is intentionally maintained in the ON state in the present embodiment. Thereby, also in the period T1, a current flows through the electric motor 10 via the power switching element Swn on the low potential side, and no current flows through the free wheel diode Fwn. This is because the amount of voltage drop when a current flows through the body diode (Fwn) is larger than the amount of voltage drop when a current flows through the power switching element Swn. Thus, by performing control (shoot-through control) to turn on both of the power switching elements Swp and Swn, the dead time period is eliminated, and thus a situation in which current flows through the body diode is avoided.

  In the period T1 in which the shoot-through control is performed, the increase rate of the current flowing through the high-potential-side power switching element Swp is limited by the input-side inductor W1, and thus gradually increases. That is, normally, in the ON state of both of the power switching elements Swp and Swn, which should be avoided until the dead time is provided, since the input-side inductor W1 is provided, the current flowing through these power switching elements Swp and Swn increases rapidly. Thus, an excessive current that causes a decrease in reliability does not flow. Here, the current ip flowing through the high-potential-side power switching element Swp is equal to the current flowing through the input-side inductor W1 and the current is flowing through the sub-switch Sws. And the input voltage VDC, “VDC / L”. As this current increases, the absolute value of the current flowing through the power switching element Swn on the low potential side decreases. That is, the current flows through the power switching element Swn on the low potential side due to the force to continue to flow the phase current due to the output side inductor (the inductor of the electric motor 10). It is covered by the current flowing through the power switching element Swp. For this reason, the absolute value of the current flowing through the power switching element Swn on the low potential side decreases.

  Then, after the absolute value of the current flowing through the low-potential-side power switching element Swn becomes zero (after the current flow direction is reversed), the low-potential-side power switching element Swn is turned off, so that the period T2 Migrate to In this case, the current flowing through the power switching element Swp on the high potential side decreases to the same amount of current as the phase current. On the other hand, the current flowing through the power switching element Swn on the low potential side decreases to zero. These reduction speeds are controlled by the switching speed (gate discharge speed) of the power switching element Swn from the on state to the off state. Here, the current flowing through the power switching element Swn immediately before the power switching element Swn on the low potential side is turned off is an amount obtained by subtracting the phase current from the current flowing through the input side inductor W1. Since this current flows through the input-side inductor W1, even if the power switching element Swn is switched to the off state, it cannot be suddenly reduced to zero, and the body diode of the main switch Swm, the sub switch Sws, and A loop circuit including the input-side inductor W1 flows. Therefore, in the period T2, the current flowing through the input-side inductor W1 and the current flowing through the sub switch Sws are not reduced. In addition, by shifting to the period T2, the voltage V2 at the connection point of the power switching elements Swp and Swn rises to the input voltage VDC.

  Thereafter, the main switch Swm is turned on to shift to the period T3. As a result, the current flowing through the body diode of the main switch Swm flows between the input terminal and the output terminal of the main switch Swm. Here, the main switch Swm is turned on after the low-potential-side power switching element Swn is switched to the off state.

  Thereafter, the sub switch Sws is turned off to shift to the period T4. As a result, the current flowing through the sub switch Sws becomes zero. As a result, a counter electromotive voltage is generated in a direction that prevents a decrease in the current flowing through the input-side inductor W1. This counter electromotive voltage is intended to cause a forward current for the diode 22 to flow through the energy supply inductor W2. For this reason, the energy stored in the input-side inductor W1 is recovered by the capacitor 24 via the energy supply inductor W2.

  And it transfers to the period T5 because the electrical energy stored in the input side inductor W1 is output to the energy supply inductor W2. In the period T5, the voltage V1 at the connection point between the sub switch Sws and the input-side inductor W1 rises to the voltage VDC of the high-voltage battery 12.

  Thereafter, the main switch Swm and the high-potential side power switching element Swp are turned off to shift to the period T6.

  According to the embodiment described in detail above, the following effects can be obtained.

  (1) The energy stored in the input-side inductor W1 provided for performing soft switching was recovered in the capacitor 24 via the energy supply inductor W2. Thereby, the energy stored in the input-side inductor W1 can be recovered by the power storage means (capacitor 24) different from the high-voltage battery 12.

  (2) An in-vehicle low voltage system in which the power switching elements Swp, Swn, the main switch Swm, and the sub switch Sws of the inverter IV constitute the in-vehicle high voltage system and the capacitor 24 is insulated from the in-vehicle high voltage system. It was supposed to be composed. Thereby, the electric power of the high voltage battery 12 which comprises a high voltage system can be supplied to the capacitor | condenser 24 which comprises a low voltage system, without passing through a dedicated converter etc.

  (3) The capacitor 24 is used as a power source for the cooling device 26 that cools the power switching elements Swp and Swn of the inverter IV, the main switch Swm, and the sub switch Sws. Thereby, when the required power of the cooling device 26 is large, a large amount of power can be supplied to the capacitor 24.

  (4) When switching the power switching element Swp on the high potential side to the on state, shoot-through control is performed to turn on both the power switching element Swn on the low potential side and the power switching element Swp on the high potential side. Further, after the total current flowing through the low potential side power switching element Swn and the free wheel diode Fwn became zero, the low potential side power switching element Swn was turned off. In other words, the low potential side power switching element Swn is turned off after the current flowing through the high potential side power switching element Swp matches the current flowing through the input side inductor W1. As a result, it is possible to preferably avoid a current from flowing through the freewheeling diode Fwn on the low potential side, and thus a surge due to the recovery current can be preferably avoided.

  (5) A MOS field effect transistor is used as the power switching element Swn. As a result, the input terminal and the output terminal are interchanged to allow current to flow bidirectionally between the pair of terminals. Therefore, it is preferable to prevent current from flowing through the freewheel diode Fwn by shoot-through control. be able to.

(Second Embodiment)
Hereinafter, the second embodiment will be described with reference to the drawings with a focus on differences from the first embodiment.

  FIG. 3 shows a system configuration according to the present embodiment. In FIG. 3, members corresponding to those shown in FIG. 1 are given the same reference numerals for convenience.

  As shown in the figure, also in the present embodiment, the first sub switch Sws1, the first input side inductor W1a, and the first main switch Swm1 are arranged between the high potential side power switching element Swp and the positive electrode of the battery 12. Connect in parallel. However, in the present embodiment, the second sub switch Sws2, the second input side inductor W1b, and the second main switch Swm2 are further connected in parallel between the low potential side power switching element Swn and the negative electrode of the battery 12. To do.

  Here, the second sub switch Sws2, the second input-side inductor W1b, and the second main switch Swm2 are recovery current countermeasure means for the freewheeling diode Fwp connected in reverse parallel to the high-potential-side power switching element Swp. . That is, when the phase current is input to the inverter IV side, when the power switching element Swn on the low potential side is turned on, the second sub switch Sws2 is turned on to perform shoot-through control. As a result, the current flowing through the high-potential side power switching element Swp before the shoot-through control gradually decreases as the current flowing through the second input-side inductor L2 gradually increases. Then, after the total current flowing through the power switching element Swp on the high potential side and the freewheel diode Fwp connected in reverse parallel thereto becomes zero, the power switching element Swp on the high potential side is turned off. Thereby, the situation where a recovery current flows can be suitably suppressed about the free wheel diode Fwp connected in antiparallel to the power switching element Swp on the high potential side.

  In addition, in the present embodiment, the electrical energy stored in each of the first input side inductor W1a and the second input side inductor W1b is output to the capacitors 24a and 24b via the first energy supply inductor W2a and the second energy supply inductor W2b. can do. The capacitors 24a and 24b are both power sources for the cooling device 26.

(Other embodiments)
Each of the above embodiments may be modified as follows.
<About the cooling device>
The cooling device is not limited to one that circulates the cooling liquid CL in the cooling passage that contacts the cooling target and performs heat exchange between the cooling target and the cooling liquid CL. For example, it may be provided with a fan that allows ambient gas to flow.

The cooling target of the cooling device is not limited to the power switching elements Swp and Swn, the main switch Swm, and the sub switch Sws (including the body diode). For example, a transformer T may be used. Further, for example, the smoothing capacitor 13 or the high voltage battery 12 which is a DC power source of the inverter IV may be used. In these cases, it is considered that the heat generation amount increases as the current flowing through the inverter IV increases. Therefore, when these are to be cooled, the effect equivalent to the effect (3) of the first embodiment can be obtained. . For example, the control apparatus 30 may be sufficient. Even in this case, when the control device 30 is arranged close to the inverter IV, etc., the greater the amount of heat generated by the inverter IV, the greater the amount of heat received by the control device 30, so the first implementation described above. The effect according to the effect of the mode (3) can be obtained.
<About power storage means>
The power storage means for recovering the energy stored in the input-side inductor W1 is not limited to the power source of the cooling device for the power switching elements Swp and Swn. For example, the low voltage battery 32 may be used. In any case, if the power storage means is a component of the low voltage system, the power of the high voltage battery 12 can be supplied to the low voltage system without using a dedicated DCDC converter or the like. In the case of the second embodiment, the output of the first energy supply inductor W2a and the output of the second energy supply inductor W2b may be used for different purposes.

Furthermore, the power storage means is not limited to that constituting a low voltage system.
<About power switching elements Swp and Swn>
The power switching elements Swp and Swn are not limited to super junction MOS field effect transistors, and may be arbitrary field effect transistors. Here, not only the N channel but also the P channel may be used.

In addition, the input terminal and the output terminal are interchanged so that the bidirectional current from one of the pair of terminals to the other and the other to the other is not limited to the one, for example, an insulated gate bipolar transistor (IGBT). Alternatively, only one direction of current may be allowed. In this case, it is desirable to connect a free wheel diode as a discrete component in reverse parallel to the IGBT. When this configuration is applied to the first embodiment, the current flowing through the freewheeling diode connected in antiparallel to the low potential side power switching element Swn by switching the high potential side power switching element Swp to the on state. It is desirable that the current is once caused to flow through the low-potential side power switching element Swn by turning off the low-potential side power switching element Swn after the time when the voltage gradually decreases to zero.
<About power conversion circuit>
The power conversion circuit is not limited to an inverter connected to a three-phase rotating machine. For example, a DC power source (battery) is connected to a series connection body of a high-potential side power switching element Swp and a low-potential side power switching element Swn. The step-down converter may be connected in parallel, and the first inductor and the capacitor are connected in parallel to the connection point of the series connection body and the negative electrode of the DC power supply. Note that in this case, the first inductor is a component of the power conversion circuit.

However, the output voltage is not limited to the input voltage or less, and may be a step-up / down chopper circuit, for example. For example, a DC power source is connected in parallel to the low potential side power switching element Swn via the first inductor, and a second inductor is provided between the high potential side switching element and the output terminal. Also good. A capacitor is provided at a pair of output terminals of the step-up / down chopper circuit.
<About the operation method of the power switching elements Swp and Swn>
The end timing of the shoot-through control is not limited to that in the above embodiments. For example, in the first embodiment, the low-potential-side power switching element Swn may be turned off when the total current flowing through the low-potential-side power switching element Swn and the freewheel diode Fwn becomes zero.

Further, the operation method of the power switching elements Swp and Swn is not limited to the one that performs shoot-through control. Even if this is not performed, the energy stored in the input-side inductor W1 can be output via the energy supply inductor W2 by opening the sub switch Sws after closing the main switch Swm.
<Others>
-The rotating machine is not limited to the main machine of a hybrid vehicle, but may be the main machine of an electric vehicle.

  The timing for turning on the sub switch Sws (Sws1, Sws2) may not be the same as the timing for turning on the power switching element Swp (Swp, Swn), for example, after that.

  In the second embodiment, the first input-side inductor W11, the first main switch Swm1, and the first sub switch Sws may be deleted. This is a configuration different from that of the first embodiment in the arm to which the recovery current countermeasure is applied.

  In each of the above embodiments, an input-side inductor, a main switch, and a sub switch may be provided for each phase.

  DESCRIPTION OF SYMBOLS 10 ... Electric motor (one embodiment of 1st inductor), 12 ... High voltage battery, 22 ... Diode, 24 ... Capacitor, 26 ... Cooling device, 30 ... Control device, IV ... Inverter, W1, W1a, W1b ... Input side inductor (One Embodiment of Second Inductor), W2, W2a, W2b... Energy Supply Inductor (One Embodiment of Third Inductor)

Claims (5)

Electric power comprising a series connection body of a high potential side switching element for connecting the first inductor to the positive electrode side of the DC power source and a low potential side switching element for connecting the first inductor to the negative electrode side of the DC power source In the conversion system,
A free wheel diode is connected in reverse parallel to one of the high potential side switching element and the low potential side switching element,
A first electric path including a second inductor and a first opening / closing means that connects the electrode side connected to the first inductor by the other switching element of the pair of electrodes of the DC power supply and the other switching element. When,
A second electric path connecting the electrode side connected to the first inductor by the other switching element of the pair of electrodes of the DC power source and the other switching element and having a second opening / closing means;
Prior to switching from the ON state of the one switching element to the ON state of the other switching element, the first opening / closing means is closed and the second opening / closing means is opened, and the second opening / closing is performed after the switching. An opening / closing operation means for closing the means and opening the first opening / closing means;
A third inductor magnetically coupled to the second inductor;
Rectifying means for setting the voltage applied to the third inductor when the first opening / closing means is switched to the open state by the opening / closing operation means as a forward voltage;
The power conversion system, wherein the third inductor is connected to power storage means different from the DC power source via the rectifying means.   2. The power conversion system according to claim 1, wherein the power storage means is a power supply of a cooling device that cools at least one of the power conversion system and the DC power supply. The high-potential side switching element and the low-potential side switching element constitute an in-vehicle high voltage system,
The power conversion system according to claim 1, wherein the power storage unit constitutes an in-vehicle low voltage system insulated from the in-vehicle high voltage system. When switching the other switching element to the on state, shoot-through control means for turning both on,
After the both are turned on by the shoot-through control means, the one switching element after the time when the total current flowing through the one switching element and the freewheel diode connected in reverse parallel to the switching element becomes zero The power conversion system according to any one of claims 1 to 3, further comprising an off operation means for turning off the element.   5. The switching device according to claim 1, wherein the one switching element allows the current to flow bidirectionally between the pair of terminals by switching between the input terminal and the output terminal. 6. The power conversion system according to item.

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